On Wed, 1 Apr 2026 13:07:33 GMT, Roman Kennke <[email protected]> wrote:
>> I analyzed the performance of Thread.setName() in response to a customer
>> workload running Cassandra, where Thread.setName() showed up (mostly because
>> of rather pathetic use of setName() from Cassanda *sigh*).
>>
>> Profiling showed that most time (around 75%) is spent in the actual syscall,
>> so there are limits on what we can do.
>>
>> I implemented the following improvements:
>> - Almost all thread names are Latin1/ASCII, and there is no need to convert
>> to UTF8 in that case. Also, the various OS APIs to set the thread name don't
>> even seem to specify the character encoding. Avoiding the UTF8 conversion
>> (if possible) brings down the length-dependent costs. In many cases we can
>> also pass down the backing array of the string and avoid copying altogether.
>> - For truncating the name on Linux to 16 chars, instead of using snprintf
>> with a pattern, we can simply stitch together the name directly (first 7
>> chars, last 6 chars, 2 dots in between), this saves ~100ns.
>>
>> In the end, we bring down performance for the small cases by ~7%, longer
>> names by ~20% and completely removed the conversion overhead that primarily
>> affected longer names.
>>
>> | Benchmark | (length) | Baseline (ns/op) | Optimized (ns/op) | Change
>> |
>>
>> |---------------|----------|------------------:|-------------------:|--------:|
>> | setName | 1 | 602.3 ± 2.0 | 561.9 ± 1.5 |
>> -6.7% |
>> | setName | 4 | 605.9 ± 2.1 | 570.2 ± 1.2 |
>> -5.9% |
>> | setName | 15 | 617.1 ± 2.7 | 570.4 ± 2.8 |
>> -7.6% |
>> | setName | 16 | 712.1 ± 6.0 | 569.4 ± 2.7 |
>> -20.0% |
>> | setName | 50 | 757.9 ± 5.2 | 566.3 ± 4.6 |
>> -25.3% |
>> | setName | 200 | 986.2 ± 2.7 | 569.9 ± 4.9 |
>> -42.2% |
>> | setNameSame | 1 | — | 7.4 ± 0.0 | —
>> |
>> | setNameSame | 4 | — | 7.4 ± 0.0 | —
>> |
>> | setNameSame | 15 | — | 7.4 ± 0.0 | —
>> |
>> | setNameSame | 16 | — | 7.4 ± 0.0 | —
>> |
>> | setNameSame | 50 | — | 7.4 ± 0.0 | —
>> |
>> | setNameSame | 200 | — | 7.4 ± 0.0 | —
>> |
>>
>>
>> Testing:
>> - [x] tier1
>> - [ ] tier2
>>
>> The failing GHA test in langtools seems unrelated.
>
> Roman Kennke has updated the pull request incrementally with one additional
> commit since the last revision:
>
> Fix MacOS build
src/hotspot/os/bsd/os_bsd.cpp line 2263:
> 2261: char buf[MAXTHREADNAMESIZE] = { 'J', 'a', 'v', 'a', ':', ' '};
> 2262: const char* prefix = "Java: ";
> 2263: const size_t prefix_len = strlen(prefix);
Suggestion:
char buf[MAXTHREADNAMESIZE] = { 'J', 'a', 'v', 'a', ':', ' '};
const size_t prefix_len = 6;
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/30374#discussion_r3025753832
